As compared with widely used amorphous silicon (a-Si), an amorphous (noncrystalline) oxide semiconductor has high carrier mobility, a high optical band gap, and film formability at low temperature and, therefore, has been highly expected to be applied for next generation displays which are required to have a large size, high resolution, and high-speed drive, resin substrates which has low heat resistance, etc.
Examples of the oxide semiconductor include In-containing amorphous oxide semiconductors (In—Ga—Zn—O, In—Zn—O, etc.), however, since these oxide semiconductors use In which is rare metal, an increase in material cost is a serious concern in mass production process. Therefore, as an oxide semiconductor containing no In, produced at low material cost, and suitable for mass production, a ZTO-based oxide semiconductor made amorphous by adding Sn to Zn has been proposed (e.g. Patent Document 1).